1. Acid & Base Applications Chapter 15

2. Review: Reaction of Weak Bases with Water
The base reacts with water, producing its conjugate acid and hydroxide ion:
CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.38 x 10-4

3. Reaction of Weak Bases with Water
The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion:
B + H2O  BH+ + OH-

4. Buffered Solutions
A solution that resists a change in pH when either hydroxide ions or protons are added.
Buffered solutions contain either:
A weak acid and its salt
A weak base and its salt

5. Examples of some Acid/Salt Buffering Pairs
The salt will contain the anion of the acid, and the cation of a strong base (NaOH, KOH)
Weak Acid
Formula
of the acid
Example of a salt of the weak acid
 Hydrofluoric 
HF 
 KF – Potassium fluoride 
 Formic 
 HCOOH 
 KHCOO – Potassium formate 
 Benzoic 
 C6H5COOH 
 NaC6H5COO – Sodium benzoate
 Acetic 
 CH3COOH 
 NaH3COO – Sodium acetate 
 Carbonic 
 H2CO3 
 NaHCO3 - Sodium bicarbonate
 Propanoic 
 HC3H5O2 
  NaC3H5O2  - Sodium propanoate
 Hydrocyanic 
 HCN 
 KCN - potassium cyanide 

6. Base/Salt Buffering Pairs
The salt will contain the cation of the base, and the anion of a strong acid (HCl, HNO3)
Base
Formula of the base
Example of a salt of the weak acid
Ammonia 
 NH3
 NH4Cl - ammonium chloride
 Methylamine
 CH3NH2
 CH3NH2Cl – methylammonium chloride
 Ethylamine
 C2H5NH2
 C2H5NH3NO3 -  ethylammonium nitrate
 Aniline
 C6H5NH2 
C6H5NH3Cl – aniline hydrochloride
 Pyridine
 C5H5N 
  C5H5NHCl – pyridine hydrochloride

7. What does a buffer do? And How?
A buffer re….

8. Titration of an Unbuffered and buffered Solution
A solution that is
0.10 M CH3COOH and
0.10 M NaCH3COO
is titrated with
0.10 M NaOH
A solution that is
0.10 M CH3COOH
is titrated with
0.10 M NaOH

9. Buffer Example Problem
A buffered solution contains 0.5 M HC2H3O2 (Ka=1.8x10-5) and 0.5M NaC2H3O2.
Find pH of the solution
major species: HC2H3O2, Na+, C2H3O2-, H2O
HC2H3O ⇄ H C2H3O2- I
0.5 C -x +x E
0.5-x x
0.5+x

10. Buffer Example (cont.)
What would happen to the pH if we added mol of solid NaOH was added to 1.0 L of this buffered solution?

11. Buffer Example (cont.) HC2H3O2 + OH-  C2H3O2- + H2O
New major species: HC2H3O2, Na+, OH-, C2H3O2-, H2O
HC2H3O2 + OH-  C2H3O2- + H2O
goes to completion because of Strong Base
HC2H3O OH  C2H3O2- + H2O I
(1.0L)(0.050M) =0.50 mol
0.010 mol C
-x= mol
+x= mol
+x=0.010 mol E
0.49 mol
0.51 mol

12. Buffer Example (cont.)
Now that we know the amount of acetic acid left, we must find out how much H+ is created from it.
HC2H3O ⇄ H C2H3O2- I
0.49
0.51 C -x +x E
0.49-x x
0.51+x

13. Henderson-Hasselbalch Equation

14. Example of Henderson-Hasselbalch Equation
Find the pH of 0.75 M HC3H5O3 (Ka=1.4x10-4) and 0.25 M NaC3H5O3
major species: HC3H5O3, Na+, C3H5O3-, H2O

15. Titrations Made Easy

16. Weak Acid/Strong Base Titration
Endpoint is above pH 7
A solution that is
0.10 M CH3COOH
is titrated with
0.10 M NaOH

17. Strong Acid/Strong Base Titration
Endpoint is at
pH 7
A solution that is
0.10 M HCl is titrated with
0.10 M NaOH

18. Strong Acid/Strong Base Titration
A solution that is
0.10 M NaOH is titrated with
0.10 M HCl
Endpoint is at
pH 7
It is important to recognize that titration curves are not always increasing from left to right.

19. Strong Acid/Weak Base Titration
A solution that is
0.10 M HCl is titrated with
0.10 M NH3
Endpoint is below
pH 7

20. Selection of Indicators

21. Some Acid-Base Indicators
pH Range in which Color Change Occurs
Color Change as pH Increases
Crystal violet Thymol blue Orange IV Methyl orange Bromcresol green Methyl red Chlorophenol red Bromthymol blue Phenol red Neutral red Thymol blue Phenolphthalein Thymolphthalein Alizarin yellow Indigo carmine
yellow to blue red to yellow red to yellow red to yellow yellow to blue red to yellow yellow to red yellow to blue yellow to red red to amber yellow to blue colourless to pink colourless to blue yellow to blue blue to yellow

22. pH Indicators and their ranges

23. Only shows where the color change occurs